Comments

The full_name_hash does not produce a value that is evenly distributed
over the lower 8 bits. This causes name hash to be unbalanced with large
number of names. There is a standard function to fold in upper bits
so use that.
This is independent of possible improvements to full_name_hash()
in future.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 28 Oct 2009 07:07:10 +0100
> You should therefore use hash_32(hash, NETDEV_HASHBITS),> not hash_long() that maps to hash_64() on 64 bit arches, which is> slower and certainly not any better with a 32bits input.
Agreed.
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On Wed, 28 Oct 2009 07:07:10 +0100
Eric Dumazet <eric.dumazet@gmail.com> wrote:
> Stephen Hemminger a écrit :> > The full_name_hash does not produce a value that is evenly distributed> > over the lower 8 bits. This causes name hash to be unbalanced with large> > number of names. There is a standard function to fold in upper bits> > so use that.> > > > This is independent of possible improvements to full_name_hash()> > in future.> > > static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)> > {> > unsigned hash = full_name_hash(name, strnlen(name, IFNAMSIZ));> > - return &net->dev_name_head[hash & ((1 << NETDEV_HASHBITS) - 1)];> > + return &net->dev_name_head[hash_long(hash, NETDEV_HASHBITS)];> > }> > > > static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)> > full_name_hash() returns an "unsigned int", which is guaranteed to be 32 bits> > You should therefore use hash_32(hash, NETDEV_HASHBITS),> not hash_long() that maps to hash_64() on 64 bit arches, which is> slower and certainly not any better with a 32bits input.
OK, I was following precedent. Only a couple places use hash_32, most use
hash_long().
Using the upper bits does give better distribution.
With 100,000 network names:
Time Ratio Max StdDev
hash_32 0.002123 1.00 422 11.07
hash_64 0.002927 1.00 400 3.97
The time field is pretty meaningless for such a small sample
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